Clinical and experimental animal studies have implicated preischemic hyperglycemia as a risk factor for increased neuronal damage after ischemic stroke. Although hyperglycemia-induced lactic acidosis under ischemia has been proposed to account for the exaggerated brain damage, the underlying molecular mechanism has not been elucidated. It has recently been demonstrated in vitro that hydroxyl free radical (OH) can be formed by the decomposition of peroxynitric acid and that the yield of hydroxyl radicals is increased in an acidic environment. The hypothesis to be tested in this project is that hydroxyl free radical production is increased in cerebral ischemia in hyperglycemic animals by reaction of nitric oxide with superoxide under hyperglycemia-induced lactic acidosis, thereby worsening the extent of ischemic brain damage. Cerebral ischemia will be induced by permanent or temporary occlusion of middle cerebral artery (MCA) in streptozotozin-induced hyperglycemic rats. The extracellular level of hydroxyl radicals will be continuously monitored in vivo during ischemia/reperfusion by intracranial microdialysis perfusion of the OH trapping agent salicylate. Non-invasive magnetic resonance imaging (MRI) techniques will be employed to monitor brain pathophysiological indicators, including neuronal damage, vasogenic edema and cerebral perfusion, during the course of ischemia/reperfusion. Intracellular pH and the distribution of high energy phosphates within the injured brain region will be measured in vivo using 31/P magnetic resonance spectroscopy (MRS). Liposome-entrapped free radical scavengers superoxide dismutase (SOD) and nitric oxide synthase inhibitor N/G-nitro- L-arginine methyl ester (L-NAME) will be administered to reduce the in vivo concentration of superoxide and nitric oxide respectively. The effects of SOD and L-NAME on the production of hydroxyl radicals and on the attenuation of MRI-measurable brain injury will be compared to the respective control groups. These studies will provide in vivo evidence linking the peroxynitric acid decomposition pathway of OH generation to the exacerbation of ischemic brain damage under hyperglycemic conditions.